1. Field of the Invention
The invention relates generally to an arrangement and a method for adaptively controlling a transducer to compensate for linear and/or nonlinear signal distortion generated by the transducer and to realize a desired overall transfer characteristic between the electric input signal and the output signal.
2. Description of the Related Art
Transducers used as actuators (loudspeakers, headphones, shakers) produce substantial linear and nonlinear distortion in the output produced by the actuator. This distortion affects the quality of the sound reproduction or impairs the efficiency of active sound attenuation systems. An electric controller connected to the input terminals of the transducer can compensate for signal distortion if the controller utilizes the inverse transfer characteristic of the transducer. Compensation for inherent nonlinearities of the transducer requires a nonlinear controller, which can be realized, e.g., by using a polynomial filter as disclosed in U.S. Pat. No. 4,709,391, or a mirror filter as disclosed in U.S. Pat. No. 5,438,625, or with static state feedback linearization as described by J. Suykens et. al, "Feedback Linearization of Nonlinear Distortion in Electrodynamic Loudspeakers," J. Audio Eng. Soc., vol. 43 (1995), pp. 690-694.
In order to cope with parameter uncertainties, the adjustment of the free parameters of the controller can be performed adaptively, as disclosed in the German patent application DE 4332804 A1. This arrangement allows the filter parameters to be determined in the normal operating mode reproducing an audio or other signals, without off-line pre-training, and adapting on-line for parameter changes caused by heating and aging. However, an adaptive controller requires information about the output signal or internal states of the transducer. The direct measurement of an acoustic or a mechanical output signal requires a precise sensor (e.g., microphone, accelerometer) which is expensive and impractical in many applications.
The German patent DE 4334040 discloses an adaptive control system which dispenses with an additional acoustic or mechanical sensor. An adaptive detector circuit estimates the velocity of the voice-coil by using the electric voltage and the current measured at the terminals of the transducer. The estimated velocity is supplied to an adaptive control filter and is used for the estimation of optimal filter parameters. The adaptive adjustment of the filter and the adaptive adjustment of the detector are two separate processes, which can be realized with different filter architectures. However, two separate adaptive systems cause high computational complexity which can not be implemented on available digital signal processors at low costs.